I can't help with the mechanic, unfortunately. Any interest in troubleshooting?
To clarify, no-start no-run, unless fed ether- does it keep running if you stop feeding it ether? Is there any fuel smell coming from the exhaust as you keep cranking?
If not, suspect fuel solenoid or IP. If yes (even poorly, like the engine isn't happy about trying to run) suspect glow-plug system.
Fuel shutoff/IP is something I've never had break (at at least, I've never had to try fixing one) but glow-plugs seem to bork around me.
Does the relay click when you push the button? If not, check that relay has power & switch is grounding the relay properly. If you modded it so the switch puts power to a grounded relay, reverse that.
Do you have 24v power to the resistor-pack side of the relay? If you don't have full voltage, check connections & fusable link. Resistor pack may have failed, but I've not seen it yet.
Do you have any power to the glow-plug side when the relay clicks? If you have nearly full battery voltage to the GP side, you need new plugs or wiring. If you have no voltage at all, you need a new relay. Anything between 6-14v is ok, higher means you may have some bad plugs.
Check the glow plugs- I've seen a truck go from All-good to all-open in a day or two from the snowball effect. If you're running a switch, consider going to 24v HMMWV plugs without the resistor.
If it cranks normally, you probably have decent batteries.
As an addendum to the usual 2 rules about CUCV's (Rubber, replace it- electrical, clean it.) I'll add check each glow plug when you can- a single failure isn't noticeable, but it'll snowball ****ed quick.